Change representation of array and slice types

[N1ark]

This was discussed IRL at some point and wanted to get it done :)
Changed representation of arrays and slices to not use TyKind::Adt, and instead just be TyKind::Array(_, _) and TyKind::Slice(_, _)

Right now tests fail because of mismatches trait clauses with arrays/slices, I'm not exactly sure what part of typecheck_and_unify.rs handles it, I tried having a look and couldn't see anything suspicious. The errors look like

error: Type error after transformations:
       Mismatched trait clause:
       expected: [@TraitClause0]: MetaSized<missing(@TypeBound(1, 0))>
            got: {built_in impl MetaSized for [u32; 2 : usize]}: MetaSized<[u32; 2 : usize]>
       Visitor stack:
         charon_lib::ast::types::TraitRef
         charon_lib::ids::index_map::IndexMap<charon_lib::ast::types::vars::TraitClauseId, charon_lib::ast::types::TraitRef>
         charon_lib::ast::types::GenericArgs
         alloc::boxed::Box<charon_lib::ast::types::GenericArgs>
         charon_lib::ast::expressions::FnPtr
         charon_lib::ast::gast::FnOperand
         charon_lib::ast::gast::Call
         charon_lib::ast::llbc_ast::StatementKind
         charon_lib::ast::llbc_ast::Statement
         alloc::vec::Vec<charon_lib::ast::llbc_ast::Statement>
         charon_lib::ast::llbc_ast::Block
         charon_lib::ast::gast::GExprBody<charon_lib::ast::llbc_ast::Block>
         charon_lib::ast::gast::Body
         charon_lib::ast::gast::FunDecl
       Binding stack (depth 1):
         0:
 --> tests/ui/simple/ptr-from-raw-parts.rs:8:27
  |
8 |     let _: *const [u32] = core::ptr::from_raw_parts(&raw const a, 2);
  |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Nadrieril]

Superceded by #958.